This invention relates to a testing method of semiconductor devices and to a display device which uses such a testing method. More particularly, the invention relates to a testing method suitable for testing such types of semiconductor devices which have at least one of the main terminals thereof isolated electrically from the outside. The present invention also specifically relates a display device of the type in which a display member can be interposed between display electrodes.
Various testing methods are known in the art to judge whether or not a transistor having at least a pair of main terminals and a control terminal, for example, a semiconductor control device has a normal ON-OFF function. Such methods are essentially based upon the method which applies a d.c. bias across the collector and emitter (or the drain and source) of the transistor, applies a current to the base (or a voltage to the gate) bias and checks the collector (or the drain) current at that time. This method can be practised extremely easily if the three terminals of the transistor can be externally accessed, but can not be carried out if any one of the terminals cannot. This is because no circuit can be formed, and an entirely novel testing method is necessary in order to test those devices whose terminals cannot be externally accessed.
The description given above will be more easily understood from FIGS. 2 and 3 of the accompanying drawings which illustrate a definite example of the semiconductor devices. FIG. 2 shows a general purpose MOS (Metal Oxide Semiconductor) transistor 1. Generally, a drain terminal 2, a source terminal 3 and a gate terminal 4 of the transistor 1 are exposed to the outside. Whether or not this transistor can be used as a switch is checked by applying a d.c. voltage across the drain and the source from outside and applying a voltage to the gate terminal 4. Since a current flows through the drain or the source in this case, the ON function can be tested. (If the MOS transistor has no switch function, the current does not flow.)
However, this method cannot be applied to those devices in which one of the ends of the switch is open and hence a connection terminal cannot be externally accessed. In the device 1 of this type, the drain terminal is open under the state 2A and no contact can be established from outside.
An example of such devices whose terminals are not externally accessed is an active matrix display device using displays such as a liquid crystal, EL or the like. An extremely large number of devices shown in FIG. 3 are integrated in such a display. An example of the testing methods of the display device is disclosed, for example, in Japanese Patent Laid-Open No. 38498/1982. This prior art technique judges the leakage of the device from the change of a stored charge quantity of a capacitor after the passage of a predetermined time and thus measures any faults of an active matrix substrate used for liquid crystal display and the address. However, the prior art technique is not free from the problem that since the test is conducted after the liquid crystal is sealed, the liquid crystal becomes useless if the transistor has a defect. In addition, the prior art is based on the premise that a circuit for the test is incorporated in the display device, so that the device area cannot be reduced easily.